1. Field of the Invention
The invention relates to precharge circuits and, more particularly, relates to precharge-circuits using microprocessor-based firing angle control circuitry.
2. Discussion of Prior Art
Precharge circuits are well known for precharging a DC bus capacitor in AC to DC converters. The bus capacitor is used to decrease the ripple voltage of an AC to DC rectifier output. The bus capacitor must be precharged before full power is applied to the converter, otherwise severe transients will occur.
In FIG. 1, a simple AC to DC converter without a precharge circuit is shown. In FIG. 2, a simple AC to DC converter with a precharge circuit is shown. In FIGS. 3 and 4, commonly used AC to DC converters with precharge circuits incorporating additional features are shown. In FIG. 5, an AC to DC converter comprised of Silicon Controlled Thyristors (SCRs) and utilizing a conventional linear phase control scheme is shown.
Referring to FIG. 1, a simple AC to DC converter 10 without a precharge circuit is shown. Converter 10 has a three phase AC supply 11 which is engaged when the utility customer closes a connect/disconnect switch 12. The AC supply flows through drive fuses 14 and is rectified by a three phase diode bridge rectifier 16. Link current flows from rectifier 16 through link inductor 18 and charges DC bus capacitor 19. A DC bus output 17 of the converter 10 is typically connected to an inverter (not shown), which converts the DC output power back to AC power.
A first problem with this converter design is that the application of full line voltage to the converter during power start-up results in a large peak surge current to charge the bus capacitor, initially at zero voltage. More specifically, the surge current can be approximated as the peak line voltage divided by the surge impedance Z.sub.o, where Z.sub.o is equal to the square root of the link inductance divided by the bus capacitance. The surge current is usually above the semiconductor diode maximum allowable rating, causing the diode to be unduly stressed and to have a decreased number of life turn-on cycles before failure. The large peak AC current will also cause unacceptable fuse fatigue and possibly cause the fuses to blow. The large precharge DC current through link inductor 18 will typically saturate the link inductor 18, which is 150% the size needed for steady state operation. The saturated link inductor impedance is reduced to an air core winding inductance value, further increasing precharge current stress. Link inductor saturation can be prevented and current stress reduced with a link inductor magnitude approximately ten times the size needed for steady state operation. However, an oversized inductor will increase the size and cost of the converter.
A second problem with this design is that the application of full line voltage to the converter during power start-up results in large voltage overshoots (approaching twice peak line voltage) in the capacitor 19. This problem is caused by the resonant nature of the inductor-capacitor combination, and is particularly severe given that the inductor-capacitor circuit is nearly undamped.
A third problem is that, in the event of a faulted DC bus, precharge operation will result in an unacceptably large current flowing into the fault. There is nothing in the line to limit the current caused by a ground fault (node 15 shorted to ground or node 13 shorted to ground) or a bus fault (node 15 shorted to node 13). The resulting link fault current will typically saturate the link inductor 18 to low inductance values. Thus, link fault current will see little impedance as it flows through the saturated link inductor directly into a ground fault or bus fault.
Referring to FIG. 2, an AC to DC converter 20 is shown which comprises a precharge circuit 23. A three phase AC supply 21 is engaged when the utility customer closes customer connect/disconnect switch 22. Current from phase A flows through a drive fuse 30a and into a three phase diode bridge rectifier 24. Link current from the rectifier 24 then flows through a DC link inductor 26 and charges a bus capacitor 28. The circuit is completed by precharge damping resistor 32 which is connected to phase C through precharge fuse 36b.
Control circuitry supply transformer 34 is connected to phases B and C through precharge fuses 36a and 36b. Transformer 34 supplies AC power to an AC to DC converter 38. Converter 38 supplies DC power to power supply 40, which outputs standard +5 v, +15 v, and -15 v voltage sources. These voltage sources are supplied to control circuitry 42. Control circuitry 42 uses differential amplifier 37 to sense when bus capacitor 28 is fully charged, and then responds by closing power contactor 44. In addition to phase A current, current from phases B and C will now flow through the rectifier 24 as well, and DC bus output 29 will be on-line.
The addition of a precharge circuit improves the performance of converter 20 as compared to converter 10. The resonant inductor-capacitor circuit is now damped by precharge resistor 32, which lessens the severity of the voltage and current transient characteristics.
With regard to current transients in particular, the addition of the precharge resistor will improve transient performance by limiting current. However, if the value of the precharge resistor is low, the majority of the peak surge current is now taken as a single pulse of current in phase A and C devices rather than being spread out over a number of 60.degree. intervals. Increasing the precharge resistor values will decrease the peak surge current and spread device heating over a number of 60.degree. charging intervals, as well as reducing the fuse 36b current rating and cost. However, a larger precharge resistor will also result in a longer time required for capacitor precharge.
Additionally, the problem incurred when operating into a faulted DC bus still exists. Finally, this circuit has a new disadvantage: additional parts are needed including power contactor 44 which is expensive and adds significant cost to the converter.
Referring to FIG. 3, a converter circuit 50 is shown which comprises a precharge circuit 55. A three phase AC supply 51 is engaged when a connect/disconnect switch 53 is closed. However, application of power through the disconnect switch 53 does not immediately charge bus capacitor 60. Instead, the customer must first depress push button 73 which is located in a converter cabinet down-line from the disconnect switch. Also, control circuitry 78 will verify that there are no faults and that phase loss relay 68 has the correct voltage. When push button 73 is depressed and there are no line faults, control circuitry 78 will cause precharge relays 62 and 66 to close.
Current from phase A will then flow through drive fuse 52a, precharge fuse 70, precharge relay 66, precharge damping resistor 64, and into a three phase diode bridge rectifier 56. Rectifier 56 converts the AC power to DC power. Link current then flows from rectifier 56 through the DC link inductor 58 and into DC bus capacitor 60. The circuit is then completed by relay 62, precharge fuse 71b, and drive fuse 52c.
A control circuitry supply transformer 72, AC to DC converter 74, and power supply 76 work to supply control circuitry 78 with power in a manner similar to the corresponding elements in converter 20.
The voltage E.sub.f across the bus capacitor 60 is measured by differential amplifier 77. When the control circuitry 78 senses that the DC bus capacitor 60 is fully charged, it will engage power contactor 54. Alternatively, a timer (not shown) which will automatically cause the contactor to close may be used. This is commonly done to avoid having to use sensing circuitry. Once the contactor 54 is closed, the DC bus will be on-line.
Converter 50 has the same transient, faulted DC bus, and power contactor problems encountered with converter 20. Converter 50 is an improvement over converter 20 in that it detects line faults before attempting to charge the DC bus capacitor.
Referring now to FIG. 4, a converter 80 is shown which comprises a precharge circuit 85. This converter design is typically used in Japan. A three phase AC supply 83 is engaged when the customer closes a connect/disconnect switch 81.
Current will flow through drive fuses 82 into a three phase diode bridge rectifier 84. During precharge operation, power contactor 88 is open. DC current flows from rectifier 84 through a precharge resistor 86, a precharge fuse 92, and a DC link inductor 90 to charge DC bus capacitor 94. Control circuitry 104 will sense when capacitor 94 is charged using differential amplifier 103 and respond by closing power contactor 88.
A control circuitry supply transformer 96 is connected to phases B and C through the fuses 98a and 98b. The control circuitry supply transformer 96, AC to DC converter 100, and power supply 102 operate to supply control circuitry 104 with power in a manner similar to the corresponding elements in precharge circuit 20.
Converter 80 has transient, faulted DC bus, and power contactor problems similar to those encountered with converters 20 and 50. Converter 80 has the additional disadvantage that it requires a large contactor rated to break DC inductive current. This requirement will further increase the cost as compared to the AC supply contactor system and is also difficult to procure at high power levels.
Referring to FIG. 5, an AC to DC converter 110 is shown which comprises a precharge circuit 115. The precharge circuit is further comprised of linear phase control electronics 134 and a three phase SCR rectifier 123 which implement a linear phase control scheme.
A three phase AC supply 112 is engaged when the utility customer closes a customer disconnect switch 114. Current from the AC supply flows through drive fuses 116a, 116b, 116c and into the three phase SCR rectifier 123. Link current from the rectifier 123 flows into a link inductor 128 and charges a DC bus capacitor 130. The SCR rectifier 123 selectively controls when current from each phase of the three phase AC supply 112 will be used to charge the bus capacitor 130. A snubber circuit comprised of a resistor 124 in series with a capacitor 126 prevents nuisance firing of the SCRs in rectifier 123.
The firing times of the SCRs in rectifier 123 are controlled by the electronics 134. The electronics 134 receive line-to-line voltage measurement inputs and a bus capacitor voltage measurement input. Line-to-line voltages are measured by differential amplifiers 132a and 132b, passed through filters 118a and 118b, and input to the electronics 134 at inputs 120a and 120b. Filters 118a and 118b are used to remove the effects of SCR commutation line notches. The voltage E.sub.f across the bus capacitor 130 is measured by differential amplifier 138, passed through filter 136, and finally input to the electronics 134. The electronics 134 outputs control signals to the SCR rectifier 123 at outputs 133a through 133f.
A control circuitry supply transformer 120 is connected to phases B and C through the fuses 119a and 119b. The control circuitry supply transformer 120, AC to DC converter 122, and power supply 124 operate to supply electronics 134 with power in a manner similar to the corresponding elements in precharge circuit 20.
A standard equation which might form the basis of a linear phase control scheme is given in equation 1. ##EQU1## where V.sub.1-1(rms) is the line to line voltage of the input
V.sub.d is the DC link voltage at the SCR rectifier output PA1 V.sub.dio is the DC link voltage at the SCR rectifier output when .alpha.=0 radians PA1 .alpha. is the SCR control firing angle in radians PA1 V(.alpha.) is an SCR control voltage which corresponds to the radian firing angle and is generated by the cosine rider scheme.
The firing angle is defined as the phase angle at which the SCR is turned on by the precharge circuit. The firing angle is equivalent to a firing time, except that it is specified in terms of the phase angle of a line-to-line voltage (and shifted by 60 degrees), rather than in terms of time. Typically, .alpha. is defined such that .alpha.=0 when the line-to-line phase angle .theta.=60.
A conventional linear cosine rider scheme can be used to control link current. For information on linear cosine rider schemes, see Richard Pearlman, "Power Electronics and Solid State Motor Control," Prentice Hall (1980) and Albert Kloss, "A Basic Guide to Power Electronics," John Wiley and Sons (1984), both of which are incorporated herein by reference. If a linear cosine rider scheme is used, then equation 1 can be implemented as given in equation 2. ##EQU2## where E.sub.f is the feedback voltage measured across the DC bus capacitor,
The SCR firing angle versus time is adjusted with V(.alpha.) by feeding back the bus capacitor voltage E.sub.f and measuring the line-to-line voltage V.sub.1-1. The resulting voltage difference between V.sub.d and E.sub.f across the link inductor limits the peak DC link current.
A major drawback to converter 110 is that the system will function properly only when the performance of its elements is linear. Thus, if the performance of one of its elements becomes non-linear, then equations 1 and 2 will no longer apply and the system will cease to function properly. Performance can become non-linear in at least three situations.
First, the link inductor 128 may saturate and its performance become non-linear. System components are commonly chosen based on optimal steady state operation. However, the minimum allowable controlled link current during precharge with this conventional control scheme is much greater than the steady-state link current, causing the link inductor to saturate. The inductor may be oversized, but this will add to the cost and size of the system, and the inductor will no longer be optimal for steady state operation. The higher precharge current also requires the input fuses 116a-116c to be increased, thereby providing a non-optimal fuse coordination for SCR short circuit protection.
A second instance where system performance is nonlinear is at the limit of link current discontinuity. Charge is preferably supplied to the capacitors over several 60 degree cycles to avoid stress problems caused by large peak currents. At the endpoint of each 60 degree cycle, however, the current is discontinuous. A linear phase control system will not be able to maintain linear operation through these discontinuities, resulting in non-controlled discontinuous current pulses of high peak magnitude.
A third instance where system performance is non-linear is when the peak magnitudes of the line-to-line voltages are varying as the capacitor is charging (e.g., as in the case of line transients on the AC supply). Once the alpha firing angle in a linear system is set, there is no more control and the system can not handle varying voltages. A linear system can not handle non-linear variations in the line-to-line voltages.
In addition to its inability to operate in a non-linear environment, the linear precharge control converter suffers from the same high peak current, non-controlled ground/bus fault current, and bus voltage overshoot problems as the converters described above.